The present invention relates to a busbar protection method, in particular, in order to protect a busbar of transmission and distribution system in electricity, it relates to a method of detecting a fault on the busbar and particularly specifying a specific fault section of the busbar.
FIG. 2 shows a part of a single line diagram in a substation illustrating a high voltage busbar in detail. The detail structure of a low voltage busbar, which is connected to low voltage transformers 1B and 2B, is omitted in this figure.
There are two busbars X and Y, and interlink lines arranged to interlink both X- and Y-busbars to be selectably connected to either one of the busbars. The transformers 1B and 2B as well as the transmission lines A and B are connected to the foregoing interlink lines and further to the X- and Y- busbars via line switches LX- and LY-, which serve to selectively connect the foregoing transformers and transmission lines to either of the X- and Y-busbars. The line switches LX- and LY- are generally called disconnecting switches. In many cases, an apparatus not illustrated in the figure is also connected to the two or double busbars in the same way.
Reference alphabets in the figure are designated as CB for the circuit breaker, as BCT for the current transformer at a side away from the busbar, as LCT for the current transformer near the busbar, as TCT for the current transformer, as X for the X-busbar, and as Y for the Y-busbar. Reference alphabets with numeral following hyphen designate locations where the apparatus is installed. However, omission of the characters following hyphen, such as the above LX-, designates general expression not related to the location where the apparatus is installed.
The lines A-1L, A-2L, B-1L and B-2L as well as the transformers 1B and 2B are connected to the double busbars via the circuit breakers CB-. The current transformers are installed on both sides of the circuit breakers CB-. The current transformers LCT-connected to the busbar serve to protect the lines or the apparatus, such as transformers, and the current transformers BCT- connected to the line or the apparatuses serve to protect the busbar. The current transformers LCT- and BCT- may be collectively connected to the busbars, the lines or the apparatus, wherein the current transformers which are installed near the busbar are designated as LCT-, and those connected to the apparatuses are designated as BCT-. As shown in the figure, other interlink lines which interlink both of the busbars are circuit breakers CB-B, LX-B and LY-B as well as CT-X and CT-Y.
Lines, such as transmission lines, and apparatus, such as transformers, are connected in general to the double busbars so as to equalize load of both of the busbars as much as possible. If the line 1L is connected to the X-busbar, for instance as shown in the following FIG. 3, then the line 2L is connected to the Y-busbar. As results, the current flowing through the circuit breaker CB-B which interlinks between the busbars is nearly zero.
There has been known a protection method of the two or double busbar system as described in the following.
FIG. 3 shows the principle of a whole protection method of the two or double busbars. In this figure, all of the secondary circuits of the current transformers BCT- installed in the lines or in the apparatus, such as transformers, are connected in parallel, and a detecting relay RY is also connected in parallel to the circuit thereof. Consequently, total of the detected current of all of the current transformers BCT- flows through the relay RY. Therefore, if fault does not occur at a busbar side relative to a position where the current transformers BCT are installed, the current flowing through the relay RY is zero. However, if fault has occurred elsewhere, a current corresponding to that flowing in the faulted point flows through the relay RY. In order to detect the fault on the busbar, the relay RY is actuated when current flowing through the relay exceeds the predetermined threshold where errors of the current transformer are taken into account.
In many cases, fault of the two or double busbars occurs on either the X-busbar or the Y-busbar, but rarely, fault occurs simultaneously on both busbars. Accordingly, it is desirable to detect the fault and identify which of the X-busbar or the Y-busbar has fault. In order to continue electric power supply, it is more advantageous to isolate the faulted busbar and to keep the sound busbar alive rather than to isolate both of the busbars. According to FIG. 4, a method to identify the faulted busbar against the sound busbar is described in the following.
As shown in FIG. 4, inputted to a busbar protection equipment 1 are data, such as the current signals outputted from the current transformers BCT- installed in the lines, apparatus and etc., the current signals outputted from the current transformers BCT- for interlinking the busbars and the output signals for indicating the switching states of the line switches LX- and LY-, i.e. "open" or "closed".
A digital processing device is built in the busbar protection equipment 1. The foregoing input signals are sampled by the digital processing device in a certain period, and, on the basis of the data thereof, digital computing is then performed according to the following calculations. Not only an instantaneous value of the current but also a value vectorically synthesized from the effective value of the currents are available in this calculation. In this figure, the current of a single phase is expressed. However, three phase currents may be handled in practical cases. EQU .SIGMA. I (the secondary current of the current transformer which are electrically connected to the LX-)&gt;K (1) EQU .SIGMA. I (the secondary current of the current transformer which are electrically connected to the LY-)&gt;K (2) EQU .SIGMA. I (the secondary current of the current transformer other than IX and IY)&gt;K (3)
wherein the .SIGMA. designates a total of the currents I, and the K designates a threshold value. The equation (1) indicates that the fault has occurred on the X-busbar, the equation (2) indicates that the fault has occurred on the Y-busbar, and the equation (3) indicates that the fault has occurred somewhere on the busbars without identifying the faulted busbar (the X-busbar or the Y-busbar).
Therefore, a trip command to the circuit breaker CB- is outputted from the busbar protection equipment 1, according to the following conditions:
(a) The trip command for the circuit breakers connected to the X-busbar is based on (3) (1) PA1 (b) The trip command for the circuit breakers connected to the Y-busbar is based on (3) (2) PA1 (c) The trip command for all of the circuit breakers is based on (3) (2) (-) (1) (-) PA1 where the indicates the logical product (and), and the (1) (-) and (2) (-) designate invalidity of equation (1) and (2) respectively.
In practice, in order to keep the apparatus sound and safe, some conditions are added to the foregoing logical product (and), such as detection of a voltage fall in the faulted phase, a zero-sequence voltage generated due to the line-to-ground fault, or an overcurrent in the busbar interlink.
However, according to the aforementioned method by which the fault is detected and protected on a basis of the busbar as a unit, it is not possible to detect the location where the fault has occurred. Therefore, this method provides no proper protection and this has been an open problem.
The present invention has been completed to eliminate the foregoing problems associated with the conventional busbar protection method.
The object of the present invention is to detect a fault which has occurred on the busbar and to specify a specific fault section of the busbar and to thus provide a more proper protection of the busbar.